In a battery powered wireless device, long battery life may perhaps be the single most important feature. To extend battery life, it is common to put the wireless device to sleep when it is not being used. However, to ensure that the user of the wireless device is not missing any incoming calls, pages, etc. that may be desirable, the wireless device must periodically wake up to check and see if it needs to process any incoming calls, pages, etc.
The wireless device would periodically wake up and detect and then decode a particular transmitted symbol, commonly referred to as a paging indicator, and then depending upon the value of the paging indicator, the wireless device could go back to sleep or continue processing information in order to receive an incoming call, page, etc. The frequency of the wireless device waking up to detect and then decode the paging indicator can be dependent upon the frequency of the paging indicator itself.
One potential source of concern involved with putting a wireless device to sleep and then waking it up may be that the internal clock of the wireless device may drift away from the system clock of the wireless communications system. This may be commonly referred to as frequency error. Clock drift may be the result of an internal clock in the wireless device not being able to keep time as accurately as the system clock. Significant clock drift may prevent the accurate detecting and decoding of the paging indicator.
A commonly used technique to help reduce the frequency error involves waking up the wireless device prior to the expected arrival time of the paging indicator and then using automatic frequency control (AFC) circuitry to reduce (or eliminate) any frequency error that may exist. Once the frequency error is reduced to acceptable levels, the paging indicator may then be accurately detected and decoded.
One disadvantage of the prior art is that the use of AFC circuitry may take a long amount of time to reduce the frequency error down to an acceptable level. While the AFC circuitry is operating, the power consumption can be large and the overall battery life of the wireless device may be shortened. Furthermore, when the AFC circuitry is running, a significant portion of the wireless device may be powered, hence adding to the power consumption. Therefore, it is desired to minimize the amount of time that the AFC circuitry (and hence the wireless device) is being powered.
A second disadvantage of the prior art is that with AFC circuitry, the operating rate of the AFC circuitry may be limited by the hardware. This means that the AFC circuitry may not be able to reduce the frequency error to an acceptable level within the allotted amount of time. This may require that the wireless device be woken up earlier, thus further reducing the battery life of the wireless device.